1. Field of the Invention
The present invention relates to an alternating current detection coil which is formed on an insulating substrate to carry out a non-contact detection of a current passing through a branch circuit in a household power distribution board and so on.
2. Description of the Related Art
A conventional air-core sensor coil used for an alternating current detection is shown in FIGS. 27 and 28 (refer to Japanese Laid-Open Patent Publication No. HEI 06-176947). In these diagrams, a sensor coil 100 comprises a double-sided multilayer board 102 (referred to as a print board hereinafter) having a circular opening 101, and a coil 103 disposed around the opening 101. The print board 102 is made of an epoxy resin including glass. The coil 103 has a conductor which is printed radially, centering around the opening 101, and the coil is formed by connecting these conductors in series through conductive connection parts which include through-holes, in other words, conducting layers formed on an inner surface of a penetration opening passing through in a thickness direction of the print board 102, that is, in an axial direction of the coil 103. The coil in the print board 102 is wound around in two directions at a certain pitch, and it comprises a coil 105, which is wound in a clockwise direction as indicated by an arrow 104 (referred to as a forward coil hereinafter), and a coil 107, which is wound in a counterclockwise direction as indicated by an arrow 106 (referred to as a backward coil). The forward coil 105 and the backward coil 107 are connected in series by connecting an end of the forward coil 105 and a leader of the backward coil 107, respectively.
In the forward coil 105, the conductors formed on the front face of the print board 102 are indicated in heavy full line, and the conductors formed on the rear face of the print board 102 are indicated in heavy broken line. In the backward coil 107, the conductors formed on the front face of the print board 102 are indicated in double full line, and the conductors formed on the rear face of the print board 102 are indicated in double broken line. On the front face and the rear face of the print board 102, the conductors in the forward coil 105 and in the backward coil 107 are alternately disposed at a certain pitch, respectively. In the forward coil 105, the conductors having different length are alternately disposed on the front face and the rear face at a certain pitch, and also in the backward coil 107, in the same manner, the conductors having different length are alternately disposed on the front face and the rear face at a certain pitch. The respective pitches of the conductors in the forward coil 105 are connected to each other on an outer side far from the opening 101, and the respective pitches of the conductors in the backward coil 107 are connected to each other on an inner side close to the opening 101. Consequently, when seeing the forward coil 105 and the backward coil 107 from a direction perpendicular to the thickness direction of the print board 102, areas of regions surrounded by the respective conductors differ according to their coil pitches. Moreover, when seeing the forward coil 105 and the backward coil 107 from the thickness direction of the print board 102 (the axial direction), area of regions surrounded by the respective conductors differ, also.
In the sensor coil 100 having such a feature, a conductor (an electrical wire) being detected is let into the opening 101, and then an induced current, which is generated by passing a magnetic flux, caused by a current passing through the conductor being detected, through a cross-section region surrounded by the conductors in the forward coil 105 and the backward coil 107, is detected. To increase the detection sensitivity, it is necessary to increase an induced electromotive force from a detecting magnetic field contributing to a detection in the forward coil 105 and the backward coil 107, and it is preferable to generate the induced electromotive force uniformly by equalizing the areas surrounded by the conductors in the forward coil 105 and the backward coil 107 in case of cutting along a surface perpendicular to the print board 102.
However, the cross-section areas surrounded by the forward coil 105 and the backward coil 107 differ according to their coil pitches, thus sensitivity of the detecting is deteriorated. Specifically, a difference of the cross-section areas surrounded by the forward coil 105 and the backward coil 107 affects in detecting an external magnetic field as the sensor coil gets closer to an electric wire, which does not need to be detected. More specifically, when the electrical wire which does not need to be detected is close to the forward coil 105 and the backward coil 107, a magnetic flux having an oblique component relatively increases compared to a case that it is positioned away. Consequently the difference in detecting the external magnetic field occurs according to the difference of the cross-section areas surrounded by the forward coil 105 and the backward coil 107, and a canceling effect against the unnecessary magnetic field is thus weakened.
Moreover, when seeing the forward coil 105 and the backward coil 107 from the thickness direction of the print board 102, a magnetic flux in the magnetic field generated from an electrical wire which is not the conductor being detected (referred to as the external magnetic field) sometimes passes through a front surface surrounded by the conductors in the forward coil 105 and the backward coil 107, other than a magnetic flux, which is to be detected, in a magnetic field generated from the conductor being detected (referred to as the detecting magnetic field). The external magnetic field is unnecessary for the original current detection, and the detected current from the external magnetic field causes a detection error. To control the detection error, the unnecessary detected current from the external magnetic field should be cancelled by equalizing the respective front surface surrounded by the forward coil 105 and the backward coil 107, each of which winds in a direction opposite to each other.
However, in the sensor coil 100 described above, the front surface area surrounded by the forward coil 105 is larger than the front surface area surrounded by the backward coil 107. Consequently, the forward coil 105 and the backward coil 107 cannot cancel out the induced current generated from the external magnetic field completely, and it is difficult to control the detection error.
There is also a sensor coil similar to the sensor coil described above, and when seeing it from an axial direction, a forward coil is formed in a sawtooth pattern, and a backward coil is formed in a triangular pattern, so that a pitch of each coil is uniformed (refer to Japanese Laid-Open Patent Publication No. 2004-87619). However, even in this sensor coil, in the same manner as the sensor coil described above, a negative effect from the external magnetic field cannot be cancelled sufficiently for the reason that the respective areas surrounded by the forward coil and the backward coil still differ.